The present invention pertains to the inspection of moving sheet material and, more particularly, to apparatus for detecting a streak extending in the machine direction of the sheet.
Optical scanners have been used to inspect sheet material moving as a continuous web in a high-speed production line at rates in excess of 100 meters per minute. The material can be, for example, paper, plastic, or metal. Such scanners may be set up to inspect the material for any number of characteristics which are measured by illuminating the material and comparing the intensity of light reflected or transmitted therefrom with threshold levels. A common type of optical scanner includes an array of photosites, each providing a pixel signal having a magnitude representing the intensity of light received from a corresponding point on the sheet material. Each photosite corresponds to the location of the point on the sheet material being inspected. The length of the array corresponds to the width of the sheet material, inspected by that array, so that the photosites provide successive sets of pixel signals, each set corresponding to a scan of the width of the sheet material, with succeeding sets progressing along the sheet in the machine direction. Such systems, which are particularly adapted to detect spots or holes in the sheet, are known as spot/hole systems.
Oftentimes, it is desired to detect persistent marks or streaks, on the sheet, extending in the machine direction. If of adequate optical contrast, such streaks will be detected by conventional inspection systems and represented as a series of spots in a machine-direction line. However, if the streak contrast is lower than the normal optical variation, or "noise", of the material being inspected, it will not be detected by presently available spot/hole detection equipment.
Using a clay coated paper manufacturing process as an example, streaks are easily produced in the clay coating. However, these streaks often alter the transmissivity of the sheet by an amount smaller than the normal point-to-point transmissivity variation of the paper, known as "optical formation". Detection of such streaks is presently accomplished by utilizing a separate set of detectors and sampling them at a low rate so as to allow a significant length of paper to be included in each sample. Formation induced transmissivity variations tend towards zero as the sample length increases, whereas the signal variation produced by the streak will persist; i.e., the formation noise average approaches zero, while the streak induced component of the signal does not. As a result, the signal representing the streak has greater contrast, allowing its detection. This method of streak detection requires: a separate timing means, a separate set of detectors, a separate means for controlling the light intensity used by those detectors, and a separate means for processing the signals. In addition to the cost of these separate items, the separate detectors occupy critical space in the process area.
It is, therefore, an object of the invention to provide an improved apparatus for optically detecting streaks in a moving sheet of material, the apparatus being effective to detect streaks when the streak contrast is lower than the normal optical variation of the material being inspected.
It is a further object of the invention to provide an improved streak detection apparatus, of the character above-described, which utilizes detectors and associated hardware already emplaced as components of a spot/hole detection system.
It is yet another object to provide a streak detection apparatus which requires a minimum of computer storage capacity and which operates in conjunction with, and at the same speed as a conventional spot/hole detection system.